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Treatment of harvest discharge from intensive shrimp ponds by settling
Aquacultural Engineering 19 (1999) 147 – 161
Treatment of harvest discharge from intensive shrimp ponds by settling D.R. Teichert-Coddington *, D.B. Rouse, A. Potts, C.E. Boyd Department of Fisheries and Allied Aquacultures, Auburn Uni6ersity, Auburn, AL 36849 -5419, USA Received 9 July 1998; accepted 16 October 1998
Abstract Effluent from intensively managed shrimp ponds was examined during harvest when ponds were drained. Concentrations of nutrients and solids in effluents were significantly higher during the final 20 cm of discharge (16% of pond volume), but greatest increases occurred during the final 5 cm of discharge (3.9% of pond volume). When the final 20 cm of pond discharge was allowed to settle, near maximum sedimentation for most variables occurred within 6 h. Settling removed total nitrogen less effectively than other nutrients. Within 6 h, 100% of settleable solids, 88% of total suspended solids, 71% of volatile solids, 63% of biochemical oxygen demand (BOD), 31% of total nitrogen and 55% of total phosphorus had sedimented from the final 20 cm of effluent. For the total pond this represented 61% settleable solids, 40% total suspended solids, 24% total volatile solids, 12% BOD, 7% total nitrogen, and 14% total phosphorus. Of the total amount removed during settling, 61% of settleable solids, between 18 and 26% of BOD, nitrogen and phosphorus, and between 34 and 45% volatile and suspended solids were found in the final 20 cm of discharge (16% of pond volume). A simple treatment of pond effluents at harvest can be effected by shunting the last 10–20% of discharge through a settling pond with no more than 6-h of residence. © 1999 Elsevier Science B.V. All rights reserved. Keywords: Drainage; Harvest discharge; Shrimp ponds
* Corresponding author. Tel.: +1-334-8449208; fax: + 1-334-8444786. 0144-8609/99/$ - see front matter © 1999 Elsevier Science B.V. All rights reserved. PII: S 0 1 4 4 - 8 6 0 9 ( 9 8 ) 0 0 0 4 7 - 8
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1. Introduction Discharges of water from shrimp farms occur during water exchange to improve the quality of water in the pond and during harvest. In the US, aquacultural discharge is regulated as a point source of pollution of receiving waters (USEPA, 1997). Internationally there is concern about the impact of shrimp farm discharge on the quality of coastal waters (Teichert-Coddington, 1995). Two general approaches may be taken to modify the environmental impact of effluents. One method is to simply reduce the quantity of effluents. Hopkins et al. (1993) studied relationships between exchange and effluent water quality in shrimp ponds stocked with 44 shrimp/m2 and with daily exchange rates of 0, 2.5, and 25%. They concluded that total discharge of nutrients, solids and biochemical oxygen demand (BOD) was reduced as the exchange rate was reduced. A reduction in water exchange allows nutrients to be assimilated in the pond rather than in receiving waters. This principle was also demonstrated in a study of channel catfish ponds. Little difference was detected in water quality between ponds that were drained annually and those that were not drained for 3 years (Seok et al., 1995). The final effluent quality of ponds drained after 3 years was not different from effluent in ponds drained after 1 year. In most cases, ponds are able to assimilate nutrients wasted as metabolites during the culture season. If excessive nutrients, such as feed, are added to ponds to the point that they cannot be assimilated, water quality will deteriorate and impair growth of the culture animal (Cole and Boyd, 1986).. A second method to modify the impact of effluents is to improve the quality of effluents before discharge. Most techniques evaluated to date involve settling ponds to remove solids, and/or constructed wetlands to filter solids and remove dissolved nutrients. Wetlands are quite effective at treating effluents, but large areas are required (Schwartz and Boyd, 1995). Schwartz and Boyd (1995) calculated that from 0.7 to 2.7 times the culture pond area would be required to treat catfish pond effluent at harvest, assuming a hydraulic residence time of 1–4 days. Settling ponds retain water long enough for sedimentation of suspended solids. The large volume and high rate of discharge from aquaculture ponds at harvest would seem to make settling ponds impractical in aquaculture. However, efficiency might be increased by discharging only the final portions of pond drainage into settling ponds. This is because concentrations of nutrients and solids in effluents tend to concentrate during the last 5 – 20% of pond discharge volume (Schwartz and Boyd 1994a; Teichert-Coddington et al., 1996). Best management practices (BMPs) were recently enacted by the Texas Natural Resource Conservation Commission (1997) for the management of aquacultural effluents. One BMP required that the final 25% of pond effluent at harvest be held 48 h prior to discharge into receiving waters unless the total suspended solids of effluents does not exceed 30 mg/l. Residence time in a settling pond directly affects the size and number of settling ponds required to treat effluents. The residence time required to settle aquacultural effluents has not been well studied. Sedimentation of effluents from channel catfish ponds was evaluated in a laboratory with synthetic
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effluents made with soils from three physiographic regions of Alabama (Boyd et al., 1998). These soils contained from 28 to 55% sand, 15 to 32% silt, and 30 to 42% clay. Results demonstrated that 75% or more of total suspended solids and total phosphorus and 40% or greater of BOD and turbidity were removed from solution within 8 h of settling. We are unaware of similar studies in saline water, but the sedimentation rate of suspended solids is more rapid in saline water because the strong ionic strength of dissolved salts neutralizes negative charges on suspended clays, silts, and colloidal humic acids. In fresh water, the repulsion of negative charges maintains particles in suspension, but flocculation occurs as salinity increases to 5 ppt (Day et al., 1989). Little field data is available on treatment of shrimp farm effluents. This study was conducted to determine how nutrients and solids are distributed in effluents during draining, and to assess effects of settling time on the quality of water discharged from intensively managed shrimp ponds.
2. Materials and methods Three ponds at the Claude Peteet Mariculture Center in Gulf Shores, Al, were stocked with 45 shrimp/m2 in May 1997. Shrimp were a mixture of Penaeus 6annamei and P. stylirostris. Ponds averaged 1094 m 2 of water surface area and 1 m depth. Average pond volume at 1 m depth was 888 m3. Pond dikes and bottoms were lined with high-density polyethylene and bottoms were covered with 20 cm of soil averaging 77.9% sand, 9.2% silt and 13.8% clay (Bill McGraw, unpublished data). Ponds were equipped with knock-down drain pipes, 20 cm in diameter, located inside a concrete harvest basin. Shrimp were fed a 35% protein pellet, twice per day. Feeding rates were based on a percentage of total biomass that declined with an increase in mean individual shrimp weight. The total daily quantity of feed added to ponds did not surpass 100 kg/ha. Ponds were aerated with 1 hp (7.5 kW/ha) aspirator pump or paddlewheel aerators to maintain dissolved oxygen concentration above 2.9 mg/l (Bill McGraw, Unpublished data). During the last month of culture, water was exchanged weekly at 25–30% of pond volume. Shrimp were cultured for 147 days. Ponds were drain-harvested by lowering the drain pipe to the floor of the catch basin. During draining, water was sampled for chemical analysis simultaneously at the pond surface above the catch basin and point of discharge when pond depth was 100, 40, 20, 10, 5, and 1 cm. Average pond volume at each depth is summarized in Table 1. Water was drained from the pond bottom through a stand pipe until the last 20 cm of depth. The remaining water was then pumped by a 20-cm diameter shrimp harvest pump to an adjacent empty pond to settle. The pump intake was placed inside the harvest basin that previously had been flushed of accumulated sediments. In the settling pond, water was sampled for chemical analysis from the pump discharge (PD) and then from near the pond surface immediately upon completion of pumping (0 h), and after 6, 12, 24, and 48 h. Pumping was completed in about 45 min. Water was sampled at the pump outlet every 2–3 min with a
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metered cup and combined in a 19-l bucket. A sample for analysis was obtained from the bucket after vigorous mixing. Water samples were analyzed for total phosphorus (TP) and total nitrogen (TN) by simultaneous persulfate oxidation followed by analysis of orthophosphate (Grasshoff et al., 1983) and nitrate (van Rijn, 1993); chlorophyll a and total ammonia nitrogen (Parsons et al., 1992); solids that settled from solution in 1 h (SS), total solids retained on a 1.2 mm filter (TSS), total volatile solids (TVS) and BOD (American Public Health Association et al., 1992). Filtered solids were washed with distilled water to remove salts prior to drying and gravimetry. Solids were fired at 510°C for 24 h to determine volatile solids. Samples for BOD analyses were diluted 2 – 3 times prior to incubation with buffered and nutrient enriched distilled water. After 2 days of incubation, the dissolved oxygen concentration was determined with a polarimetric YSI sensor for BOD bottles. Afterwards, the contents were oxygenated to saturation with pure oxygen and returned for 3 more days of incubation. The reported BOD was a sum of determinations at day 2 and day 5 for a total of 5 days incubation. Data collected during drainage were analyzed by 2 factor analysis of variation (ANOVA): factors were location of sampling (bottom or surface) and pond water depth (100, 40, 20, 10, 5 and 1 cm). Data collected during the settling pond study were analyzed by one-way ANOVA, with settling time as the factor. Significant differences among means were detected with Student–Newman–Keuls pair-wise comparison (a = 0.05). Statistical analyses were accomplished using software by Gagnon et al. (1991).
3. Results
3.1. Drainage Salinity ranged between 15.5 and 16.5 ppt. No significant differences in concentrations of nutrients and solids were observed during draining until the final 20 cm of water depth (Table 2; Fig. 1). Total ammonia nitrogen was significantly higher during the last 20 cm of drainage compared with the first 80 cm of drainage. Mean concentrations of TN and TP increased as drainage proceeded, but concentrations Table 1 Mean volume of water contained in trapezoidal shaped ponds at different depths Pond depth (cm)
Volume (m3)
% Full pond
100 40 20 10 5 1
888 305 144 70 35 7
100 34.4 16.3 7.9 3.9 0.8
549.1 509.7 529.4a 586.7 543.1 564.9a 629.4 608.0 618.7a 694.4 662.3 678.3a
B S Mean B S Mean B S Mean B S Mean
20
10
5
0 s
50.7 37.9 44.3a
44.7 35.3 40.0ab
35.6 31.3 33.5ab
32.6 25.0 28.8b
27.7 28.2 27.9b
s
1.82 1.52 1.67a
1.56 1.32 1.44a
1.12 0.74 0.93b
0.88 0.70 0.79bc
0.61 0.55 0.58c
0.55 0.52 0.53c
Total phosphorus (mg/l)
ns
4.26 4.04 4.15a
4.00 2.64 3.32ab
2.88 2.70 2.79ab
3.23 2.33 2.78ab
2.06 1.53 1.80b
1.72 1.42 1.57b
Total nitrogen (mg/l)
ns
2.52 2.28 2.40a
2.35 2.33 2.34a
1.96 2.46 2.21a
2.60 1.98 2.30a
0.81 0.84 0.83b
0.62 0.56 0.59b
Total ammonia (mg/l)
hs
31.8 11.2 21.5a
24.0 5.1 14.5ac
7.5 2.0 4.7b
6.3 1.1 3.7bc
1.1 0.7 0.9bd
0.7 0.1 0.4bd
Settleable solids (ml/l)
ns
3671 1904 2788a
1488 1738 1613ab
617 216 417b
624 290 457b
182 183 183b
202 160 181b
ns
713 413 563a
351 385 368ab
168 84 126b
156 74 115b
67 73 70b
92 83 88b
Total solids Volatile (mg/l) solids (mg/l)
ns
2958 1491 2225a
1137 1352 1245ab
449 133 291b
468 216 342b
115 110 113b
110 77 93b
Ash (mg/l)
a Means within a column followed by the same letter are not significantly different. s, significant (PB0.05); hs, highly significant (PB0.01); ns, not significant (P\0.05).
ns
499.7 504.1 501.9a
B S Mean
40
Bottom vs surface
444.2 418.3 431.2a
B S Mean
100
30.5 30.7 30.6b
Chlorophyll BOD a (mg/l) (mg/l)
Sample location
Water depth (cm)
Table 2 Mean water quality at the bottom (B) and surface (S) of shrimp ponds at different water depths during harvesta D.R. Teichert-Coddington et al. / Aquacultural Engineering 19 (1999) 147–161 151
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Fig. 1. Mean concentrations ( 9S.D.) of nutrients and solids at the water surface and point of discharge when water depth was 100, 40, 20, 10, 5, and 1 cm.
were significantly higher only during the final 5 cm of water depth. Chlorophyll a tended to increase as drainage proceeded, but differences over time were not significant. Biochemical oxygen demand, SS, TSS, TVS, and ash were also significantly higher during the final 5 cm of pond depth. Concentrations of some variables were significantly affected by depth of sampling. Total phosphorus, TN and SS were significantly more concentrated in the discharge than at the surface of the pond. Significant differences between surface and bottom samples were not found for other variables. No interaction between sampling depth and stage of pond drainage were found for any variable indicat-
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ing that concentrations of a given variable at the pond surface and discharge did not change relative to each other as water level decreased. The final 20 cm of discharge accounted for only 16% of the total pond volume (Table 1). However, of the total nutrients in the pond, 24–26% of TN and TP, 34% of TVS, 45% of TSS and 61% of SS were discharged in the final 20 cm of water (Table 4). In terms of total pond nutrient mass settling, the final 20 cm of discharge removed 7% of TN, 12% of BOD, 14% of TP, 24% of TVS, 40% of TSS, and 61% of SS.
3.2. Settling Within 6 h, more than 55% of TP and BOD, 70% of TVS, 88% of TSS, and almost 100% of settleable solids had sedimented from the final 20 cm of discharge (Table 3; Fig. 2). Concentrations of nutrients and solids after 48 h of settling were not significantly different than those after 6 h of settling. Settleable solids, TSS, TVS, ash, BOD, and TP settled almost immediately, because concentrations during pumping were significantly greater than concentrations in the settling pond immediately following pumping. Mean TN concentrations decreased by 34% during 48 h of settling, but differences over time were not significant.
Fig. 1. (Continued)
a
(10.4) (0.1) (0.0) (0.0) (0.0)
990a 280b 117b 110b 79b 98b (28.3) (11.8) (11.1) (7.9) (9.9)
Total suspended solids (mg/l) 235a 95b 68b 66b 38b 45b
(40.6) (29.1) (28.3) (16.1) (19.3)
Total volatile solids (mg/l)
Means in the same column followed by the same letter are not significantly different (PB0.05).
(94.0) (69.0) (75.5) (72.2) (66.2)
7.8a 0.8b 0.0b 0.0b 0.0b 0.0b
4.35a 4.09a 3.00a 3.28a 3.14a 2.88a
1.16a 0.69b 0.52b 0.50b 0.47b 0.44b
Pumping 0h 6h 12 h 24 h 48 h (59.7) (44.8) (43.2) (40.8) (38.4)
Settleable solids (ml/l)
Total phosphorus Total nitrogen (mg/l) (mg/l)
Variable
755a 185b 49b 44b 41b 52b
(24.5) (6.4) (5.8) (5.4) (6.9)
Ash (mg/l)
45.0a 18.3b 16.6b 13.8b 19.9b 30.4b
(40.7) (36.9) (30.6) (44.2) (67.6)
BOD (mg/l)
Table 3 Mean concentrations (% of initial concentration during pumping) of nutrients and solids in the final 20 cm of discharge from shrimp ponds during pumping, immediately after pumping (0 h), and after 6, 12, 24, and 48 h of settlinga
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Fig. 2. Mean concentrations ( 9 S.D.) of nutrients and solids in a settling pond during pumping into the pond (P), upon termination of pumping (0 h), and after 6, 12, 24, and 48 h of residence.
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4. Discussion Discharge at harvest includes sediment and nutrients that have accumulated during the culture period. Pond waters include a flocculent layer of organic matter that forms on the pond bottom from dead plankton, uneaten feed, and culture animal waste (Boyd, 1995). As pond levels decline, fish or shrimp become concentrated and suspend the floc by their activity, causing it to be entrained in effluents. In a study of tilapia pond bottom sediment, carbon constituted 1.80% of the top 5 cm 1 day before harvest. Immediately following harvest, carbon content had decreased to 1.65% indicating that 0.15% of the carbon was lost through drainage of the pond (Ayub et al., 1993); no seines were used and ponds were not flushed with water. Seining, and flushing to maintain high dissolved oxygen concentrations during harvest, can greatly increase the suspension of sediments in discharge water. In ponds in Honduras stocked at 7 – 10 shrimp/m2, there was a significant inverse relationship between pond volume at drainage and concentrations of TP, TN, dissolved inorganic nitrogen, filterable reactive phosphate and settleable solids (PB 0.05) (Teichert-Coddington et al., 1996). The increase in nutrient concentrations was greatest during the final 12% of water volume when ponds were flushed with water to assist with harvest. The soils of these ponds contained from 21 to 33% sand, 29 to 47% silt, and 20 to 45% clay (Munsiri et al., 1996). In channel catfish ponds 50% of nitrogen, phosphorus, and BOD were discharged in the last 15–20% of effluent volume, and 50% of settleable solids were discharged during the last 5% of effluent (Schwartz and Boyd, 1994a). Concentrations of nutrients and solids increased sharply when seining to remove fish commenced. The results of these studies suggest that settling to remove suspended solids from harvest effluents would be most effective on the final 10–20% of discharge. In the current study, quality of discharge was generally similar in the upper 80 cm of pond volume, but deteriorated during the final 20 cm when water was pumped out of the catch basin and shrimp were concentrated and removed. Mean total ammonia concentration almost tripled when pumping began. The noise and disturbance caused by installing and operating the pump presumably agitated the shrimp causing them to resuspend sediment and release ammonia. Ammonia is produced in pond sediment primarily by bacterial mineralization of organic matter. Resuspension of sediment would release ammonia trapped in sediment pore water and allow for desorption of weakly bound ammonium on soil particles (Simon, 1989; Reddy et al., 1996). Ammonia desorption occurs within minutes and can contribute higher concentrations of ammonia to the water column than diffusion from pore water. Greatest changes in water quality were noted during the final 5 cm of pond water discharge. During the final stage, the water level had became low enough to excoriate the pond bottom as it flowed to the drain, shrimp were visibly exited, and harvesters entered the pond to rake shrimp towards the harvest basin. These combined activities produced a slurry of water and pond sediments that raised concentrations of solids 15 (volatile solids) to 30 (suspended solids) times higher, and phosphorus and nitrogen more than 2.5 times than those found in surface
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water when draining commenced. Of the suspended solids discharged with the final 5 cm of water, 78 – 80% was inorganic matter (ash), and the remainder was organic (volatile solids). By comparison, 51% of suspended solids was inorganic matter when the ponds were full. Most of the solids settled quickly indicating that they would not have been discharged without vigorous agitation of the pond bottom to suspend the heavy mineral particles. Best management practices to reduce discharge of solids at harvest should incorporate techniques that minimize disturbance of the pond bottom by seining, raking, and flushing. Well sloped and graded bottoms to promote good drainage, and catch basins to help concentrate shrimp before removal are pond construction practices that would reduce bottom disturbance during harvest. Solids, phosphorus, nitrogen, and BOD tended to concentrate at the bottom rather than at the surface of ponds, but differences between mean bottom and surface concentrations were only significant for BOD, phosphorus and settleable solids. The heavier mineral particles settled to the bottom entraining adsorbed nutrients like phosphorus and organic particles. Dissolved nutrients like ammonium that have weak attraction for soil particles, did not concentrate at the pond bottom with soil particles. Settling the final 20 cm of pond effluent for up to 6 h removed significant quantities of nutrients and solids. Settling periods of up to 48 h did not result in additional sedimentation. Almost all (99.9%) SS and 88.2% of TSS were removed within 6 h. Of the mineral fraction or ash, 93.6% was removed. Organic matter also settled out with the mineral fraction, removing about 71% of the volatile solids and 63% of BOD. The organic matter must have been physically associated with the mineral fraction, because both fractions displayed the same pattern of settling. Phosphorus must also have been associated with the mineral fraction, probably adsorbed on soil particles or as a precipitate, because concentrations decreased concomitant with a decrease in suspended solids. A similar rate of settlement was observed in laboratory studies of synthetic solutions of pond effluents (Boyd et al., 1998). In that study, turbid solutions were made with soils from catfish ponds located in three different physiographic regions of Alabama. Within 8 h, 75% or more of the total suspended solids and total phosphorus, and 40% or greater of BOD were removed by settling. A longer residence time did not result in greater removal of any measured substance. Total nitrogen tended to decrease with residence time, but not significantly. Most of the nitrogen must have been dissolved, or an element of plankton that did not settle. Ammonia was the dominant inorganic form of nitrogen found in these ponds (J. Couch, Unpublished data). During drainage, total ammonia nitrogen concentrations increased almost 3-fold with the last 20 cm of water drainage compared to concentrations at 40 and 100 cm of water depth. The ammonia probably diffused from the pond bottom sediment disturbed by the wind, activity of concentrated shrimp, and water movement. During drainage, TN concentrations increased in direct response to increasing ammonia until the last 5 cm of water. Thereafter, total ammonia stabilized but TN continued to increase, probably from particulate and dissolved organic matter in slurry that formed at the end of pond drainage.
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Concentrations of phosphorus and solids after 6 h of settling were similar to concentrations in full ponds prior to harvest, indicating that sedimentation was complete. Although TN appeared to attenuate during 48 h of settling, concentrations never returned to pre-harvest levels probably because of high concentrations of dissolved inorganic nitrogen. Boyd et al. (1998) also found that sedimentation was less effective at removing ammonia than other variables. Attenuation of TN might have resulted from removal of ammonia by nitrification in pond soil and water column. Pumping the final 20 cm of water mixed and oxygenated sediment and ammonia. During residence time in the settling pond, a breeze was strong enough to keep the water surface choppy. The combination of high oxygen and ammonia concentrations is conducive to rapid nitrification of ammonia by aerobic bacteria (Fry, 1987). Underneath the sediment surface where oxygen diffusion is slow, conditions would likely become anaerobic again quite rapidly. Denitrification of nitrate is an anaerobic process and enhanced by high nitrate concentrations (Cerco, 1989). The combination of aerated sediment to enhance nitrification and subsequent denitrification in anaerobic subsurfaces should lead to a loss of nitrogen during sedimentation (Hargreaves and Tucker, 1996). Biochemical oxygen demand of discharge water is important to the ecology of receiving waters because large quantities of high BOD water may result in critically low oxygen conditions. Data from the current study indicated that settling harvest water for 6 h removed 63% of BOD, but after water cleared of settleable solids, BOD began to increase again probably from phytoplankton growth. Chlorophyll a was not monitored in settling ponds so a direct indication of phytoplankton biomass was unavailable. However, settling ponds turned green after 24–48 h, and the BOD concentrations increased from a mean low point after 12 h to similar levels measured in full ponds prior to harvest. A phytoplankton bloom would have increased BOD. Sedimentation therefore removed BOD as particulate organic matter in the short term, but longer term BOD of effluent may not have been reduced by sedimentation because of autochthonous organic matter production in settling ponds. The proportions of total pond mass for each variable found in the final 20 cm of harvest effluent were 23.3% for BOD, 29.6% for TN, 32.2% for TP, 54.2% for TSS, and 68.4% for SS (Table 3). On a whole pond basis, sedimentation of the final 20 cm of harvest effluent for 6 h removed 7.4% of TN, 11.5% of BOD, 14.2% of TP, 39.8% of TSS, and 61.1% of SS. Mean concentrations of these variables after 6 h of sedimentation were similar to concentrations in full ponds, indicating that proportions would probably have changed little if effluent from the first 80 cm of pond water had also been allowed to settle. Mean concentrations of nutrients and solids allowable in pond discharge by most state regulatory agencies were reported as 30 mg/l BOD, 0.17 mg/l total phosphorus, 30 mg/l total suspended solids, and 3.3 ml/l settleable solids (Schwartz and Boyd, 1994b). Results from this study indicate that concentrations of TP (0.52 mg/l), and TSS (117 mg/l) after 6 h of settling were greater than these standards, but BOD (16.6 mg/l) and SS (0 ml/l) were less.
Total in pond
Mass
Chlorophyll a (g) 434.3 BOD5 (kg) 26.4 Total phosphorus (kg) 0.58 Total nitrogen (kg) 1.77 Total ammonia (kg) 0.98 Settleable solids (l) 1,889 Total suspended solids (kg) 287 Total volatile solids (kg) 91 Ash (kg) 195
Variable
4891 297.1 6.55 19.91 10.98 21 272 3230 1030 2200
Per ha-m
79.7 4.8 0.15 0.42 0.31 1.2 130 31 99
In final 20 cm of discharge 18.4 18.2 25.6 23.7 32.2 61.2 45.2 33.5 50.6
Percentage of total mass in final 20 cm
– 63.1 55.2 31.0 – 99.9 88.2 70.9 93.6
Percentage of settled from final 20 cm
– 11.5 14.2 7.4 – 61.1 39.8 23.8 47.4
Percentge of total pond mass settled from final 20 cm
Table 4 Mean mass of nutrients and solids measured in full ponds, discharge from the final 20 cm of ponds during harvest, and the proportions removed from ponds during settling
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Sedimentation was most efficient for inorganic suspended solids (ash) and least efficient for nitrogen. On a per ha/m (10 000 m3) basis of pond effluent, sedimentation removed 1.3 m3 of SS and 1285 kg of TSS, 68% of which were inorganic. Removal of suspended solids, particularly the portion that will settle rapidly, is important in clear receiving water because sediments may smother benthic habitat. In many cases, shrimp farms are located on estuaries that are already turbid from river borne sediment or sediment suspended by tidal flow (Teichert-Coddington, 1995), and the sediment entering from an aquacultural facility provide a minor contribution to overall sediment loading. The removal of nitrogen, BOD, and phosphorus by sedimentation was less efficient than removal of solids, but still effective. For every hectare-meter of effluent, sedimentation removed 1.5 kg of nitrogen, 34.2 kg of BOD and 0.9 kg of phosphorus. Considering that no more than 6 h of residence time was necessary for sedimentation, one strategically located pond could be used to settle effluent from a number of production ponds. Of the methods available for controlling discharge water quality, sedimentation may be the easiest to implement. More field studies on sedimentation should be done on effluents from ponds built on other soil types, particularly those with higher clay content. Settling times less than 6 h should be evaluated. Other simple methods of sedimentation, such as discharging effluent in well vegetated drainage canals, should be investigated.
Acknowledgements Bruno Giri and Bill McGraw assisted in the laboratory. This research was supported in part by a grant from the Mississippi–Alabama Sea Grant Program.
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Gagnon, J., Haycock, K.A., Roth, J.M., Plamondon, J., Carroll, M., Feldman, D.S., Hofmann, R., Simpson, J., 1991. SuperANOVA. Berkeley, CA, Abacus Concepts. Grasshoff, K., Ehrhardt, M., Kremling, K., 1983. Methods of Seawater Analysis. Verlag Chemie: Weinheim, p. 419. Hargreaves, J.A., Tucker, C.S., 1996. Evidence for control of water quality in channel catfish Ictalurus punctatus ponds by phytoplankton biomass and sediment oxygenation. J. World Aquacult. Soc. 27 (1), 21 –29. Hopkins, J.S., Hamilton, R.D.I., Sandifer, P.A., Browdy, C.L., Stokes, A.D., 1993. Effect of water exchange rate on production, water quality, effluent characteristics and nitrogen budgets of intensive shrimp ponds. J. World Aquacult. Soc. 24 (3), 304 – 320. Munsiri, P., Boyd, C.E., Teichert-Coddington, D., Hajek, B. F., 1996. Texture and chemical composition of soils from shrimp ponds near Choluteca, Honduras. Aquacult. Int. 4, 157 – 168. Parsons, T.R., Maita, Y., Lalli, C.M., 1992. A Manual of Chemical and Biological Methods for Seawater Analysis. Pergamon Press, New York, p. 173. Reddy, K.R., Fisher, M.M., Ivanoff, D., 1996. Resuspension and diffusive flux of nitrogen and phosphorus in a hypereutrophic lake. J. Environ. Quality 25, 363 – 371. Schwartz, M.F., Boyd, C.E., 1994. Channel catfish pond effluents. Progr. Fish-Culturist 56 (4), 273 – 281. Schwartz, M.F., Boyd, C.E., 1994. Effluent quality during harvest of channel catfish from watershed ponds. Progr. Fish-Culturist 56, 25–32. Schwartz, M.F., Boyd, C. E., 1995. Constructed wetlands for treatment of channel catfish pond effluents. Progr. Fish-Culturist 57, 255–266. Seok, K., Leonard, S., Boyd, C.E., Schwartz, M.F., 1995. Water quality in annually drained and undrained channel catfish ponds over a three-year period. Progr. Fish-Culturist 57 (1), 52 – 58. Simon, N.S., 1989. Nitrogen cycling between sediment and the shallow-water column in the transition zone of the Potomac River and estuary. II. The role of wind-driven resuspension and adsorbed ammonium. Estuarine Coastal Shelf Sci. 28, 531 – 547. Teichert-Coddington, D.R., 1995. Estuarine water quality and sustainable shrimp culture in Honduras. In: S. Hopkins and C. Browdy (Eds.), Proc. Special Session on Shrimp Farming, Aquaculture ’95, San Diego, CA, World Aquaculture Society, Baton Rouge, LA, p. 253. Teichert-Coddington, D.R., Martinez, D., Ramı´rez, E., 1996. Characterization of shrimp farm effluents in Honduras and chemical budgets of selected nutrients. In: H. Egna, B. Goetze, D. Burke, M. McNamara and D. Clair (Eds.), Thirteenth Annual Administrative Report, Pond Dynamics/Aquaculture Collaborative Research Program, 1995, Office of International Research & Development, Oregon State University, Corvallis, OR. Texas Natural Resource Conservation Commission, 1997. Title 30 TAC Chapter 321, Subchapter O-Discharges from aquaculture production facilities. Texas Natural Resource Conservation Commission, TX. United States Environmental Protection Agency, 1997. The code of federal regulation. 40 CFR, Part 122.24. United States Environmental Protection Agency, Washington, D.C. van Rijn, J., 1993. Methods to Evaluate Water Quality in Aquaculture. Rehovot, Israel, Faculty of Agriculture, The Hebrew University of Jerusalem.
Treatment of harvest discharge from intensive shrimp ponds by settling D.R. Teichert-Coddington *, D.B. Rouse, A. Potts, C.E. Boyd Department of Fisheries and Allied Aquacultures, Auburn Uni6ersity, Auburn, AL 36849 -5419, USA Received 9 July 1998; accepted 16 October 1998
Abstract Effluent from intensively managed shrimp ponds was examined during harvest when ponds were drained. Concentrations of nutrients and solids in effluents were significantly higher during the final 20 cm of discharge (16% of pond volume), but greatest increases occurred during the final 5 cm of discharge (3.9% of pond volume). When the final 20 cm of pond discharge was allowed to settle, near maximum sedimentation for most variables occurred within 6 h. Settling removed total nitrogen less effectively than other nutrients. Within 6 h, 100% of settleable solids, 88% of total suspended solids, 71% of volatile solids, 63% of biochemical oxygen demand (BOD), 31% of total nitrogen and 55% of total phosphorus had sedimented from the final 20 cm of effluent. For the total pond this represented 61% settleable solids, 40% total suspended solids, 24% total volatile solids, 12% BOD, 7% total nitrogen, and 14% total phosphorus. Of the total amount removed during settling, 61% of settleable solids, between 18 and 26% of BOD, nitrogen and phosphorus, and between 34 and 45% volatile and suspended solids were found in the final 20 cm of discharge (16% of pond volume). A simple treatment of pond effluents at harvest can be effected by shunting the last 10–20% of discharge through a settling pond with no more than 6-h of residence. © 1999 Elsevier Science B.V. All rights reserved. Keywords: Drainage; Harvest discharge; Shrimp ponds
* Corresponding author. Tel.: +1-334-8449208; fax: + 1-334-8444786. 0144-8609/99/$ - see front matter © 1999 Elsevier Science B.V. All rights reserved. PII: S 0 1 4 4 - 8 6 0 9 ( 9 8 ) 0 0 0 4 7 - 8
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1. Introduction Discharges of water from shrimp farms occur during water exchange to improve the quality of water in the pond and during harvest. In the US, aquacultural discharge is regulated as a point source of pollution of receiving waters (USEPA, 1997). Internationally there is concern about the impact of shrimp farm discharge on the quality of coastal waters (Teichert-Coddington, 1995). Two general approaches may be taken to modify the environmental impact of effluents. One method is to simply reduce the quantity of effluents. Hopkins et al. (1993) studied relationships between exchange and effluent water quality in shrimp ponds stocked with 44 shrimp/m2 and with daily exchange rates of 0, 2.5, and 25%. They concluded that total discharge of nutrients, solids and biochemical oxygen demand (BOD) was reduced as the exchange rate was reduced. A reduction in water exchange allows nutrients to be assimilated in the pond rather than in receiving waters. This principle was also demonstrated in a study of channel catfish ponds. Little difference was detected in water quality between ponds that were drained annually and those that were not drained for 3 years (Seok et al., 1995). The final effluent quality of ponds drained after 3 years was not different from effluent in ponds drained after 1 year. In most cases, ponds are able to assimilate nutrients wasted as metabolites during the culture season. If excessive nutrients, such as feed, are added to ponds to the point that they cannot be assimilated, water quality will deteriorate and impair growth of the culture animal (Cole and Boyd, 1986).. A second method to modify the impact of effluents is to improve the quality of effluents before discharge. Most techniques evaluated to date involve settling ponds to remove solids, and/or constructed wetlands to filter solids and remove dissolved nutrients. Wetlands are quite effective at treating effluents, but large areas are required (Schwartz and Boyd, 1995). Schwartz and Boyd (1995) calculated that from 0.7 to 2.7 times the culture pond area would be required to treat catfish pond effluent at harvest, assuming a hydraulic residence time of 1–4 days. Settling ponds retain water long enough for sedimentation of suspended solids. The large volume and high rate of discharge from aquaculture ponds at harvest would seem to make settling ponds impractical in aquaculture. However, efficiency might be increased by discharging only the final portions of pond drainage into settling ponds. This is because concentrations of nutrients and solids in effluents tend to concentrate during the last 5 – 20% of pond discharge volume (Schwartz and Boyd 1994a; Teichert-Coddington et al., 1996). Best management practices (BMPs) were recently enacted by the Texas Natural Resource Conservation Commission (1997) for the management of aquacultural effluents. One BMP required that the final 25% of pond effluent at harvest be held 48 h prior to discharge into receiving waters unless the total suspended solids of effluents does not exceed 30 mg/l. Residence time in a settling pond directly affects the size and number of settling ponds required to treat effluents. The residence time required to settle aquacultural effluents has not been well studied. Sedimentation of effluents from channel catfish ponds was evaluated in a laboratory with synthetic
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effluents made with soils from three physiographic regions of Alabama (Boyd et al., 1998). These soils contained from 28 to 55% sand, 15 to 32% silt, and 30 to 42% clay. Results demonstrated that 75% or more of total suspended solids and total phosphorus and 40% or greater of BOD and turbidity were removed from solution within 8 h of settling. We are unaware of similar studies in saline water, but the sedimentation rate of suspended solids is more rapid in saline water because the strong ionic strength of dissolved salts neutralizes negative charges on suspended clays, silts, and colloidal humic acids. In fresh water, the repulsion of negative charges maintains particles in suspension, but flocculation occurs as salinity increases to 5 ppt (Day et al., 1989). Little field data is available on treatment of shrimp farm effluents. This study was conducted to determine how nutrients and solids are distributed in effluents during draining, and to assess effects of settling time on the quality of water discharged from intensively managed shrimp ponds.
2. Materials and methods Three ponds at the Claude Peteet Mariculture Center in Gulf Shores, Al, were stocked with 45 shrimp/m2 in May 1997. Shrimp were a mixture of Penaeus 6annamei and P. stylirostris. Ponds averaged 1094 m 2 of water surface area and 1 m depth. Average pond volume at 1 m depth was 888 m3. Pond dikes and bottoms were lined with high-density polyethylene and bottoms were covered with 20 cm of soil averaging 77.9% sand, 9.2% silt and 13.8% clay (Bill McGraw, unpublished data). Ponds were equipped with knock-down drain pipes, 20 cm in diameter, located inside a concrete harvest basin. Shrimp were fed a 35% protein pellet, twice per day. Feeding rates were based on a percentage of total biomass that declined with an increase in mean individual shrimp weight. The total daily quantity of feed added to ponds did not surpass 100 kg/ha. Ponds were aerated with 1 hp (7.5 kW/ha) aspirator pump or paddlewheel aerators to maintain dissolved oxygen concentration above 2.9 mg/l (Bill McGraw, Unpublished data). During the last month of culture, water was exchanged weekly at 25–30% of pond volume. Shrimp were cultured for 147 days. Ponds were drain-harvested by lowering the drain pipe to the floor of the catch basin. During draining, water was sampled for chemical analysis simultaneously at the pond surface above the catch basin and point of discharge when pond depth was 100, 40, 20, 10, 5, and 1 cm. Average pond volume at each depth is summarized in Table 1. Water was drained from the pond bottom through a stand pipe until the last 20 cm of depth. The remaining water was then pumped by a 20-cm diameter shrimp harvest pump to an adjacent empty pond to settle. The pump intake was placed inside the harvest basin that previously had been flushed of accumulated sediments. In the settling pond, water was sampled for chemical analysis from the pump discharge (PD) and then from near the pond surface immediately upon completion of pumping (0 h), and after 6, 12, 24, and 48 h. Pumping was completed in about 45 min. Water was sampled at the pump outlet every 2–3 min with a
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metered cup and combined in a 19-l bucket. A sample for analysis was obtained from the bucket after vigorous mixing. Water samples were analyzed for total phosphorus (TP) and total nitrogen (TN) by simultaneous persulfate oxidation followed by analysis of orthophosphate (Grasshoff et al., 1983) and nitrate (van Rijn, 1993); chlorophyll a and total ammonia nitrogen (Parsons et al., 1992); solids that settled from solution in 1 h (SS), total solids retained on a 1.2 mm filter (TSS), total volatile solids (TVS) and BOD (American Public Health Association et al., 1992). Filtered solids were washed with distilled water to remove salts prior to drying and gravimetry. Solids were fired at 510°C for 24 h to determine volatile solids. Samples for BOD analyses were diluted 2 – 3 times prior to incubation with buffered and nutrient enriched distilled water. After 2 days of incubation, the dissolved oxygen concentration was determined with a polarimetric YSI sensor for BOD bottles. Afterwards, the contents were oxygenated to saturation with pure oxygen and returned for 3 more days of incubation. The reported BOD was a sum of determinations at day 2 and day 5 for a total of 5 days incubation. Data collected during drainage were analyzed by 2 factor analysis of variation (ANOVA): factors were location of sampling (bottom or surface) and pond water depth (100, 40, 20, 10, 5 and 1 cm). Data collected during the settling pond study were analyzed by one-way ANOVA, with settling time as the factor. Significant differences among means were detected with Student–Newman–Keuls pair-wise comparison (a = 0.05). Statistical analyses were accomplished using software by Gagnon et al. (1991).
3. Results
3.1. Drainage Salinity ranged between 15.5 and 16.5 ppt. No significant differences in concentrations of nutrients and solids were observed during draining until the final 20 cm of water depth (Table 2; Fig. 1). Total ammonia nitrogen was significantly higher during the last 20 cm of drainage compared with the first 80 cm of drainage. Mean concentrations of TN and TP increased as drainage proceeded, but concentrations Table 1 Mean volume of water contained in trapezoidal shaped ponds at different depths Pond depth (cm)
Volume (m3)
% Full pond
100 40 20 10 5 1
888 305 144 70 35 7
100 34.4 16.3 7.9 3.9 0.8
549.1 509.7 529.4a 586.7 543.1 564.9a 629.4 608.0 618.7a 694.4 662.3 678.3a
B S Mean B S Mean B S Mean B S Mean
20
10
5
0 s
50.7 37.9 44.3a
44.7 35.3 40.0ab
35.6 31.3 33.5ab
32.6 25.0 28.8b
27.7 28.2 27.9b
s
1.82 1.52 1.67a
1.56 1.32 1.44a
1.12 0.74 0.93b
0.88 0.70 0.79bc
0.61 0.55 0.58c
0.55 0.52 0.53c
Total phosphorus (mg/l)
ns
4.26 4.04 4.15a
4.00 2.64 3.32ab
2.88 2.70 2.79ab
3.23 2.33 2.78ab
2.06 1.53 1.80b
1.72 1.42 1.57b
Total nitrogen (mg/l)
ns
2.52 2.28 2.40a
2.35 2.33 2.34a
1.96 2.46 2.21a
2.60 1.98 2.30a
0.81 0.84 0.83b
0.62 0.56 0.59b
Total ammonia (mg/l)
hs
31.8 11.2 21.5a
24.0 5.1 14.5ac
7.5 2.0 4.7b
6.3 1.1 3.7bc
1.1 0.7 0.9bd
0.7 0.1 0.4bd
Settleable solids (ml/l)
ns
3671 1904 2788a
1488 1738 1613ab
617 216 417b
624 290 457b
182 183 183b
202 160 181b
ns
713 413 563a
351 385 368ab
168 84 126b
156 74 115b
67 73 70b
92 83 88b
Total solids Volatile (mg/l) solids (mg/l)
ns
2958 1491 2225a
1137 1352 1245ab
449 133 291b
468 216 342b
115 110 113b
110 77 93b
Ash (mg/l)
a Means within a column followed by the same letter are not significantly different. s, significant (PB0.05); hs, highly significant (PB0.01); ns, not significant (P\0.05).
ns
499.7 504.1 501.9a
B S Mean
40
Bottom vs surface
444.2 418.3 431.2a
B S Mean
100
30.5 30.7 30.6b
Chlorophyll BOD a (mg/l) (mg/l)
Sample location
Water depth (cm)
Table 2 Mean water quality at the bottom (B) and surface (S) of shrimp ponds at different water depths during harvesta D.R. Teichert-Coddington et al. / Aquacultural Engineering 19 (1999) 147–161 151
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Fig. 1. Mean concentrations ( 9S.D.) of nutrients and solids at the water surface and point of discharge when water depth was 100, 40, 20, 10, 5, and 1 cm.
were significantly higher only during the final 5 cm of water depth. Chlorophyll a tended to increase as drainage proceeded, but differences over time were not significant. Biochemical oxygen demand, SS, TSS, TVS, and ash were also significantly higher during the final 5 cm of pond depth. Concentrations of some variables were significantly affected by depth of sampling. Total phosphorus, TN and SS were significantly more concentrated in the discharge than at the surface of the pond. Significant differences between surface and bottom samples were not found for other variables. No interaction between sampling depth and stage of pond drainage were found for any variable indicat-
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ing that concentrations of a given variable at the pond surface and discharge did not change relative to each other as water level decreased. The final 20 cm of discharge accounted for only 16% of the total pond volume (Table 1). However, of the total nutrients in the pond, 24–26% of TN and TP, 34% of TVS, 45% of TSS and 61% of SS were discharged in the final 20 cm of water (Table 4). In terms of total pond nutrient mass settling, the final 20 cm of discharge removed 7% of TN, 12% of BOD, 14% of TP, 24% of TVS, 40% of TSS, and 61% of SS.
3.2. Settling Within 6 h, more than 55% of TP and BOD, 70% of TVS, 88% of TSS, and almost 100% of settleable solids had sedimented from the final 20 cm of discharge (Table 3; Fig. 2). Concentrations of nutrients and solids after 48 h of settling were not significantly different than those after 6 h of settling. Settleable solids, TSS, TVS, ash, BOD, and TP settled almost immediately, because concentrations during pumping were significantly greater than concentrations in the settling pond immediately following pumping. Mean TN concentrations decreased by 34% during 48 h of settling, but differences over time were not significant.
Fig. 1. (Continued)
a
(10.4) (0.1) (0.0) (0.0) (0.0)
990a 280b 117b 110b 79b 98b (28.3) (11.8) (11.1) (7.9) (9.9)
Total suspended solids (mg/l) 235a 95b 68b 66b 38b 45b
(40.6) (29.1) (28.3) (16.1) (19.3)
Total volatile solids (mg/l)
Means in the same column followed by the same letter are not significantly different (PB0.05).
(94.0) (69.0) (75.5) (72.2) (66.2)
7.8a 0.8b 0.0b 0.0b 0.0b 0.0b
4.35a 4.09a 3.00a 3.28a 3.14a 2.88a
1.16a 0.69b 0.52b 0.50b 0.47b 0.44b
Pumping 0h 6h 12 h 24 h 48 h (59.7) (44.8) (43.2) (40.8) (38.4)
Settleable solids (ml/l)
Total phosphorus Total nitrogen (mg/l) (mg/l)
Variable
755a 185b 49b 44b 41b 52b
(24.5) (6.4) (5.8) (5.4) (6.9)
Ash (mg/l)
45.0a 18.3b 16.6b 13.8b 19.9b 30.4b
(40.7) (36.9) (30.6) (44.2) (67.6)
BOD (mg/l)
Table 3 Mean concentrations (% of initial concentration during pumping) of nutrients and solids in the final 20 cm of discharge from shrimp ponds during pumping, immediately after pumping (0 h), and after 6, 12, 24, and 48 h of settlinga
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155
Fig. 2. Mean concentrations ( 9 S.D.) of nutrients and solids in a settling pond during pumping into the pond (P), upon termination of pumping (0 h), and after 6, 12, 24, and 48 h of residence.
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4. Discussion Discharge at harvest includes sediment and nutrients that have accumulated during the culture period. Pond waters include a flocculent layer of organic matter that forms on the pond bottom from dead plankton, uneaten feed, and culture animal waste (Boyd, 1995). As pond levels decline, fish or shrimp become concentrated and suspend the floc by their activity, causing it to be entrained in effluents. In a study of tilapia pond bottom sediment, carbon constituted 1.80% of the top 5 cm 1 day before harvest. Immediately following harvest, carbon content had decreased to 1.65% indicating that 0.15% of the carbon was lost through drainage of the pond (Ayub et al., 1993); no seines were used and ponds were not flushed with water. Seining, and flushing to maintain high dissolved oxygen concentrations during harvest, can greatly increase the suspension of sediments in discharge water. In ponds in Honduras stocked at 7 – 10 shrimp/m2, there was a significant inverse relationship between pond volume at drainage and concentrations of TP, TN, dissolved inorganic nitrogen, filterable reactive phosphate and settleable solids (PB 0.05) (Teichert-Coddington et al., 1996). The increase in nutrient concentrations was greatest during the final 12% of water volume when ponds were flushed with water to assist with harvest. The soils of these ponds contained from 21 to 33% sand, 29 to 47% silt, and 20 to 45% clay (Munsiri et al., 1996). In channel catfish ponds 50% of nitrogen, phosphorus, and BOD were discharged in the last 15–20% of effluent volume, and 50% of settleable solids were discharged during the last 5% of effluent (Schwartz and Boyd, 1994a). Concentrations of nutrients and solids increased sharply when seining to remove fish commenced. The results of these studies suggest that settling to remove suspended solids from harvest effluents would be most effective on the final 10–20% of discharge. In the current study, quality of discharge was generally similar in the upper 80 cm of pond volume, but deteriorated during the final 20 cm when water was pumped out of the catch basin and shrimp were concentrated and removed. Mean total ammonia concentration almost tripled when pumping began. The noise and disturbance caused by installing and operating the pump presumably agitated the shrimp causing them to resuspend sediment and release ammonia. Ammonia is produced in pond sediment primarily by bacterial mineralization of organic matter. Resuspension of sediment would release ammonia trapped in sediment pore water and allow for desorption of weakly bound ammonium on soil particles (Simon, 1989; Reddy et al., 1996). Ammonia desorption occurs within minutes and can contribute higher concentrations of ammonia to the water column than diffusion from pore water. Greatest changes in water quality were noted during the final 5 cm of pond water discharge. During the final stage, the water level had became low enough to excoriate the pond bottom as it flowed to the drain, shrimp were visibly exited, and harvesters entered the pond to rake shrimp towards the harvest basin. These combined activities produced a slurry of water and pond sediments that raised concentrations of solids 15 (volatile solids) to 30 (suspended solids) times higher, and phosphorus and nitrogen more than 2.5 times than those found in surface
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water when draining commenced. Of the suspended solids discharged with the final 5 cm of water, 78 – 80% was inorganic matter (ash), and the remainder was organic (volatile solids). By comparison, 51% of suspended solids was inorganic matter when the ponds were full. Most of the solids settled quickly indicating that they would not have been discharged without vigorous agitation of the pond bottom to suspend the heavy mineral particles. Best management practices to reduce discharge of solids at harvest should incorporate techniques that minimize disturbance of the pond bottom by seining, raking, and flushing. Well sloped and graded bottoms to promote good drainage, and catch basins to help concentrate shrimp before removal are pond construction practices that would reduce bottom disturbance during harvest. Solids, phosphorus, nitrogen, and BOD tended to concentrate at the bottom rather than at the surface of ponds, but differences between mean bottom and surface concentrations were only significant for BOD, phosphorus and settleable solids. The heavier mineral particles settled to the bottom entraining adsorbed nutrients like phosphorus and organic particles. Dissolved nutrients like ammonium that have weak attraction for soil particles, did not concentrate at the pond bottom with soil particles. Settling the final 20 cm of pond effluent for up to 6 h removed significant quantities of nutrients and solids. Settling periods of up to 48 h did not result in additional sedimentation. Almost all (99.9%) SS and 88.2% of TSS were removed within 6 h. Of the mineral fraction or ash, 93.6% was removed. Organic matter also settled out with the mineral fraction, removing about 71% of the volatile solids and 63% of BOD. The organic matter must have been physically associated with the mineral fraction, because both fractions displayed the same pattern of settling. Phosphorus must also have been associated with the mineral fraction, probably adsorbed on soil particles or as a precipitate, because concentrations decreased concomitant with a decrease in suspended solids. A similar rate of settlement was observed in laboratory studies of synthetic solutions of pond effluents (Boyd et al., 1998). In that study, turbid solutions were made with soils from catfish ponds located in three different physiographic regions of Alabama. Within 8 h, 75% or more of the total suspended solids and total phosphorus, and 40% or greater of BOD were removed by settling. A longer residence time did not result in greater removal of any measured substance. Total nitrogen tended to decrease with residence time, but not significantly. Most of the nitrogen must have been dissolved, or an element of plankton that did not settle. Ammonia was the dominant inorganic form of nitrogen found in these ponds (J. Couch, Unpublished data). During drainage, total ammonia nitrogen concentrations increased almost 3-fold with the last 20 cm of water drainage compared to concentrations at 40 and 100 cm of water depth. The ammonia probably diffused from the pond bottom sediment disturbed by the wind, activity of concentrated shrimp, and water movement. During drainage, TN concentrations increased in direct response to increasing ammonia until the last 5 cm of water. Thereafter, total ammonia stabilized but TN continued to increase, probably from particulate and dissolved organic matter in slurry that formed at the end of pond drainage.
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Concentrations of phosphorus and solids after 6 h of settling were similar to concentrations in full ponds prior to harvest, indicating that sedimentation was complete. Although TN appeared to attenuate during 48 h of settling, concentrations never returned to pre-harvest levels probably because of high concentrations of dissolved inorganic nitrogen. Boyd et al. (1998) also found that sedimentation was less effective at removing ammonia than other variables. Attenuation of TN might have resulted from removal of ammonia by nitrification in pond soil and water column. Pumping the final 20 cm of water mixed and oxygenated sediment and ammonia. During residence time in the settling pond, a breeze was strong enough to keep the water surface choppy. The combination of high oxygen and ammonia concentrations is conducive to rapid nitrification of ammonia by aerobic bacteria (Fry, 1987). Underneath the sediment surface where oxygen diffusion is slow, conditions would likely become anaerobic again quite rapidly. Denitrification of nitrate is an anaerobic process and enhanced by high nitrate concentrations (Cerco, 1989). The combination of aerated sediment to enhance nitrification and subsequent denitrification in anaerobic subsurfaces should lead to a loss of nitrogen during sedimentation (Hargreaves and Tucker, 1996). Biochemical oxygen demand of discharge water is important to the ecology of receiving waters because large quantities of high BOD water may result in critically low oxygen conditions. Data from the current study indicated that settling harvest water for 6 h removed 63% of BOD, but after water cleared of settleable solids, BOD began to increase again probably from phytoplankton growth. Chlorophyll a was not monitored in settling ponds so a direct indication of phytoplankton biomass was unavailable. However, settling ponds turned green after 24–48 h, and the BOD concentrations increased from a mean low point after 12 h to similar levels measured in full ponds prior to harvest. A phytoplankton bloom would have increased BOD. Sedimentation therefore removed BOD as particulate organic matter in the short term, but longer term BOD of effluent may not have been reduced by sedimentation because of autochthonous organic matter production in settling ponds. The proportions of total pond mass for each variable found in the final 20 cm of harvest effluent were 23.3% for BOD, 29.6% for TN, 32.2% for TP, 54.2% for TSS, and 68.4% for SS (Table 3). On a whole pond basis, sedimentation of the final 20 cm of harvest effluent for 6 h removed 7.4% of TN, 11.5% of BOD, 14.2% of TP, 39.8% of TSS, and 61.1% of SS. Mean concentrations of these variables after 6 h of sedimentation were similar to concentrations in full ponds, indicating that proportions would probably have changed little if effluent from the first 80 cm of pond water had also been allowed to settle. Mean concentrations of nutrients and solids allowable in pond discharge by most state regulatory agencies were reported as 30 mg/l BOD, 0.17 mg/l total phosphorus, 30 mg/l total suspended solids, and 3.3 ml/l settleable solids (Schwartz and Boyd, 1994b). Results from this study indicate that concentrations of TP (0.52 mg/l), and TSS (117 mg/l) after 6 h of settling were greater than these standards, but BOD (16.6 mg/l) and SS (0 ml/l) were less.
Total in pond
Mass
Chlorophyll a (g) 434.3 BOD5 (kg) 26.4 Total phosphorus (kg) 0.58 Total nitrogen (kg) 1.77 Total ammonia (kg) 0.98 Settleable solids (l) 1,889 Total suspended solids (kg) 287 Total volatile solids (kg) 91 Ash (kg) 195
Variable
4891 297.1 6.55 19.91 10.98 21 272 3230 1030 2200
Per ha-m
79.7 4.8 0.15 0.42 0.31 1.2 130 31 99
In final 20 cm of discharge 18.4 18.2 25.6 23.7 32.2 61.2 45.2 33.5 50.6
Percentage of total mass in final 20 cm
– 63.1 55.2 31.0 – 99.9 88.2 70.9 93.6
Percentage of settled from final 20 cm
– 11.5 14.2 7.4 – 61.1 39.8 23.8 47.4
Percentge of total pond mass settled from final 20 cm
Table 4 Mean mass of nutrients and solids measured in full ponds, discharge from the final 20 cm of ponds during harvest, and the proportions removed from ponds during settling
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Sedimentation was most efficient for inorganic suspended solids (ash) and least efficient for nitrogen. On a per ha/m (10 000 m3) basis of pond effluent, sedimentation removed 1.3 m3 of SS and 1285 kg of TSS, 68% of which were inorganic. Removal of suspended solids, particularly the portion that will settle rapidly, is important in clear receiving water because sediments may smother benthic habitat. In many cases, shrimp farms are located on estuaries that are already turbid from river borne sediment or sediment suspended by tidal flow (Teichert-Coddington, 1995), and the sediment entering from an aquacultural facility provide a minor contribution to overall sediment loading. The removal of nitrogen, BOD, and phosphorus by sedimentation was less efficient than removal of solids, but still effective. For every hectare-meter of effluent, sedimentation removed 1.5 kg of nitrogen, 34.2 kg of BOD and 0.9 kg of phosphorus. Considering that no more than 6 h of residence time was necessary for sedimentation, one strategically located pond could be used to settle effluent from a number of production ponds. Of the methods available for controlling discharge water quality, sedimentation may be the easiest to implement. More field studies on sedimentation should be done on effluents from ponds built on other soil types, particularly those with higher clay content. Settling times less than 6 h should be evaluated. Other simple methods of sedimentation, such as discharging effluent in well vegetated drainage canals, should be investigated.
Acknowledgements Bruno Giri and Bill McGraw assisted in the laboratory. This research was supported in part by a grant from the Mississippi–Alabama Sea Grant Program.
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